A plasma display panel (herein after referred to as a PDP) can achieve higher definition and have a larger screen. Thus, a television screen using a PDP approx. 65 inch in diagonal is commercially available. Recently, with advancement of application of PDPs to high definition televisions having the number of scanning lines twice as many as conventional televisions compliant with the National Television System Committee (NTSC) system, PDPs containing no lead to address environmental issues have been required.
A PDP is basically made of a front panel and a rear panel. The front panel includes a glass substrate made of sodium borosilicate glass by the float method, display electrodes that are made of stripe-like transparent electrodes and bus electrodes formed on the principle surface of the glass substrate on one side thereof, a dielectric layer covering the display electrodes and working as a capacitor, and a protective layer that is made of magnesium oxide (MgO) formed on the dielectric layer. On the other hand, the rear panel is made of a glass substrate, stripe-like address electrodes formed on the principle surface of the glass substrate on one side thereof, a primary dielectric layer covering the address electrodes, barrier ribs formed on the primary dielectric layer, and phosphor layers formed between the respective barrier ribs and emitting light in red, green, or blue.
The front panel and rear panel are hermetically sealed with the electrode-forming sides thereof faced with each other. A Ne—Xe discharge gas is charged in the discharge space partitioned by the barrier ribs, at a pressure ranging from 400 to 600 Torr. For a PDP, selective application of image signal voltage to the display electrodes makes the electrodes discharge. Then, the ultraviolet light generated by the discharge excites the respective phosphor layers so that they emit light in red, green, or blue to display color images.
Silver electrodes are used for the bus electrodes in the display electrodes to ensure electrical conductivity thereof. Low-melting glass essentially consisting of lead oxide is used for the dielectric layer. The examples of a lead-free dielectric layer addressing recent environmental issues are disclosed in Japanese Patent Unexamined Publication Nos. 2003-128430, 2002-053342, 2001-048577, and H09-050769.
As described above, an increasing number of PDPs is applied to high definition televisions having the number of scanning lines at least twice as many as conventional NTSC-compliant televisions.
Such compliance with high definition increases the numbers of scanning lines and display electrodes, and decreases the spacing between the display electrodes. These changes increase silver ions diffused into the dielectric layer and glass substrate, from the silver electrodes constituting the display electrodes. When the silver ions diffuse into the dielectric layer and glass substrate, the silver ions are reduced by alkali metal ions in the dielectric layer, and bivalent tin ions contained in the glass substrate, thus forming silver colloids. These colloids cause a yellowing phenomenon in which the dielectric layer or glass substrate strongly colors into yellow or brown. Additionally, the silver oxide reduced generates oxygen, thus bubbles in the dielectric layer.
Thus, an increase in the number of scanning lines more conspicuously yellows the glass substrate and generates bubbles in the dielectric layer, thus considerably degrading the image quality and causing insulation failures in the dielectric layer.
However, in the examples of the conventional lead-free dielectric layer proposed to address environmental issues, the yellowing phenomenon and insulation failures of the dielectric layer cannot be inhibited at the same time.